The present disclosure relates to a thin film transistor. In particular, the thin film transistor comprises a substrate and an oriented zinc oxide semiconductor layer.
Zinc oxide (ZnO) is known as a channel semiconductor in thin film transistors (“TFTs”). It is readily available and can be processed at ambient temperatures. It also has very high electron mobility (bulk mobility as high as 155 cm2/V·sec and field effect mobility of 70 cm2/V·sec), is environmentally stable, has a large band gap, is non-toxic, and inexpensive.
However, the method by which the semiconductor is fabricated will affect the mobility of the ZnO semiconductor layer. A ZnO semiconductor having high mobility (5-20 cm2/V·sec) is generally made only through radio-frequency magnetron sputtering. Such equipment is expensive and leads to high production costs. In another approach, ZnO precursors were used, then processed to form a ZnO semiconductor layer. However, this approach requires an annealing step at a temperature of 400-550° C. Such temperatures are not suitable for substrates which deform at lower temperatures, like polymeric substrates, such as for example polyester, polycarbonate, polyimide films or sheets. ZnO semiconductors have also been made at low (ambient) temperatures using ZnO nanoparticles or nanorods in solution. However, such semiconductors have low mobility (˜0.6 cm2/V·sec).
Zinc oxide thin film crystal usually has a Wurtzite structure (hexagonal symmetry) with lattice parameters a=3.2960 and c=5.2065 Å. The orientation of the zinc oxide can be analyzed using for example x-ray diffraction (XRD) technique. For randomly oriented zinc oxide crystals three peaks can be observed with d-spacing distance of d=2.81, 2.60, 2.48 Å for (100), (002), and (101) plane, respectively, by using Cu Kα radiation (λ 1.5418 Å). The intensity ratios of these peaks in a randomly oriented zinc oxide powder sample are respectively about I(100)/I(002)/I(101)=57/44/100 (intensities are obtained from ICDD/JCPDS card No. 36-1451 provided by The International Centre for Diffraction Data®). For randomly oriented zinc oxide crystals, the percentage of the intensity of the (002) peak relative to the sum of intensities of (100), (002), and (101) peak, I(100)+I(002)+I(101), I(002)/[I(100)+I(002)+I(101)]×100%, is about 22%±2%.
The following documents provide additional background information:    E. Fortunato et al., “Fully Transparent ZnO Thin-Film Transistor Produced at Room Temperature,” Adv. Mater., Vol. 17, No. 5, pp. 590-594 (Mar. 8, 2005).    T. E. Park et al., “Structural and Optical Properties of ZnO Thin Films Grown by RF Magnetron Sputtering on Si Substrates,” J. Korean Phys. Soc., Vol. 45, pp. S697-S700 (December 2004).    B. J. Norris et al., “Spin coated zinc oxide transparent transistors,” J. Phys. D: Appl. Phys., Vol. 36, pp. L105-L107 (2003).    B. Sun et al., “Solution-Processed Zinc Oxide Field-Effect Transistors Based on Self-Assembly of Colloidal Nanorods,” Nano Lett., Vol. 5, No. 12, pp. 2408-2413 (2005).    Y. Takahashi et al, “Photoconductivity of Ultrathin Zinc Oxide Films,” Jpn. J. Appl. Phys., Vol. 33, pp. 6611-6615 (1994).    D. Bao et al., “Sol-gel derived c-axis oriented ZnO thin films,” Thin Solid Films, Vol. 312, pp. 37-39 (1998).    M. Ohyama et al., “Preparation of ZnO Films with Preferential Orientation by Sol-Gel Method,” J. Cer. Soc. Jpn., Vol. 104, pp. 296-300 (1996).    S. Fujihara et al., “Crystallization behavior and origin of c-axis orientation in sol-gel-derived ZnO:Li thin films on glass substrates,” Appl. Sur. Sci., Vol. 180, pp. 341-350 (20011)    K. Nishio et al., “Preparation of highly oriented thin film exhibiting transparent conduction by the sol-gel process,” J. Mater. Sci., Vol. 31, pp. 3651-3656 (1996).
TFTs are generally composed of, on a substrate, an electrically conductive gate, source and drain electrodes, an electrically insulating gate dielectric layer which separated the gate electrode from the source and drain electrodes, and a semiconducting layer which is in contact with the gate dielectric layer and bridges the source and drain electrodes.